1. Field of the Invention
This invention relates to the reproduction of digital video data from a record medium and, more particularly, to recovering such data, which is recorded in D-1 format, with a minimal amount of memory devices but with little likelihood of distortion in the ultimately reproduced video picture due to mixing of data from different fields, even when the recorded digital video data is played back with special effects, such as reverse mode, fast forward mode, etc.
2. Description of the Prior Art
It is desirable to digitize conventional television signals in order to improve the fidelity and quality thereof, exploit special processing techniques and utilize conventional data processing circuitry and software to enhance the video picture which ultimately is reproduced. Once digitized, the video signal should be capable of recording; and digital video tape recording (DVTR) recently has been introduced. In an effort to standardize DVTR formats, a so-called 4:2:2 recording scheme has been adopted; and such 4:2:2 DVTR format also is known as the D-1 format.
Because of the enormous quantity of digital information that is produced by digitizing a conventional television signal, use of the conventional recording technique currently employed in analog video recording is not practical. That is, there simply is too much data for the recording of one complete field interval in a single track on magnetic tape. Thus, although the technique of recording slant tracks by rotating heads across magnetic tape continues to be used in digital video recording, the D-1 format contemplates the recording of one field interval in several tracks. For example, when recording an NTSC signal (also known as a 525/60 video signal because 525 lines of video information are present in each frame and 60 field intervals are produced each second), ten successive tracks are used to record a single field. If a pair of recording heads is used to record the digital video signals (as is used to record analog video signals), the heads will trace alternate tracks across the video tape, thus requiring them to rotate at extremely high speeds in order to record ten tracks in a field interval. Clearly, the mechanical and data processing speeds at which the heads and recording circuitry must operate are far too great for recording digital video signals with only two heads.
To overcome this drawback, plural tracks are recorded in parallel, as by using plural recording heads which simultaneously scan the magnetic tape. Likewise, during a playback operation, plural reproducing heads (which may differ from the recording heads) simultaneously scan the parallel tracks which were recorded. To provide accurate tracking of the playback heads during both normal and special effects modes, dynamic tracking of a type similar to that used in analog video recorders is provided. One example of a rotary head assembly proposed previously for DVTR applications and having dynamic tracking capability is illustrated in FIG. 1.
As shown in FIG. 1, a rotary drum or head wheel 1 has recording and playback heads supported thereon and is adapted to rotate such that the heads scan parallel slant tracks across a magnetic tape 2 wrapped about the drum. During a recording operation, the tape is driven in the direction V.sub.+ while the drum rotates in the direction CL at a rotational speed .omega.. Two pairs of recording heads R(A), R(B) and R(C), R(D) are mounted on drum 1, with the pairs spaced apart from each other by 180.degree.. Preferably, separate heads are used to play back digital video signals that had been recorded previously; and two pairs of playback heads are provided P(A), P(B) and P(C), P(D), with these pairs being spaced apart by 180.degree.. For dynamic tracking, the playback heads are mounted on movable support elements, such as bimorph elements; and, as illustrated, playback heads P(A) and P(B) are mounted on bimorph support 3, whereas playback heads P(C) and P(D) are mounted on bimorph support 4. As is known from analog dynamic tracking, the bimorph supports are movable in a direction perpendicular to the plane of the drawings so as to position the playback heads mounted thereon over the center of the tracks being scanned. For convenience, the playback heads are referred to hereafter merely as heads A, B, C and D.
The D-1 format of DVTR is described in greater detail in "Introduction to the 4:2:2 Digital Video Tape Recorder" by Stephen Gregory, Pentech Press, London, 1988, but for the purpose of understanding the problems associated with DVTR which are overcome by the present invention, a brief description of the relevant D-1 format now will be described with reference to FIG. 2A which illustrates schematically the manner in which a video field is recorded. A given slant track is divided into two video portions, each containing digital video data, these two video portions being separated by an audio portion which contains digital audio data. As shown to the right of the schematically illustrated video tape of FIG. 2A, an audio portion 2b serves to separate video portions 2a and 2c. It is preferred to position the audio portion in the vicinity of the longitudinal axis of the magnetic tape because tracking errors of the playback heads generally are minimized at that location; and errors in audio information which may be attributed to tracking errors are more perceptible than errors which may be present in the video data. Thus, to minimize a viewer's perception of interference, the audio portion 2b is disposed generally in the central portion of the tape.
An understanding of the manner in which the video data is derived and recorded in the D-1 format will best be appreciated by referring to FIG. 3 which schematically illustrates three successive video fields 5a, 5b and 5c, respectivley. For convenience, field 5a may be thought of as an even field, field 5b may be thought of as an odd field and field 5c may be thought of as an even field. Although each field in the NTSC system is comprised of 262.5 line intervals, it is appreciated that approximately 250 line intervals contain active (or useful) video information. The action portion of each field interval is divided into five segments, each segment being comprised of 50 line intervals. These five segments are identified as segments 0, 1, 2, 3 and 4. In FIG. 3, an odd field is identified with a parenthetical "1" and an even field is identified with a parenthetical "0". Thus, even field 5a is comprised of segments 0(0), 1(0), 2(0), 3(0) and 4(0) and odd field 5b is comprised of segments 0(1), 1(1), 2(1), 3(1) and 4(1).
The D-1 format records a field interval in ten tracks. Accordingly, each segment of a field is recorded in two tracks. To achieve this, each segment is divided into four sectors, identified as sectors 0, 1, 2 and 3, and two sectors are recorded in each track. To minimize gross errors that may be due to dropout or the like, adjacent sectors as well as adjacent segments are not recorded in the same track. Consequently, and as shown in FIG. 2A, the individual sectors of each individual segment are recorded in the illustrated format. It is seen that one sector is recorded in each video portion of a track and, for convenience, each video portion is illustrated with a sector number, a segment number and an indication of whether the field which contains that segment is even (0) or odd (1). Furthermore, each sector (that is, each video portion) contains, in addition to digital video data, identifying data referred to generally as identification addresses, or ID addresses formed as follows: EQU ID Address=[Sector No., Segment No. (Field No.)]
wherein the sector number may be from 0 to 3, the segmented number may be from 0 to 4 and the field number may be from 0 to 3. It is appreciated that the field number thus may be represented by a 2-bit signal with the least significant bit identifying the field as odd or even and with the more significant bit identifying the frame which contains that field.
A more detailed representation of the digital data which consitutes a recorded sector (or video portion) on magnetic tape 2 is illustrated in FIG. 2B. Each sector includes a preamble followed by 160 synchronizing blocks, the latter containing the digital video data. Each sector concludes with a postamble which, for simplification, is not illustrated. As shown, the preamble includes a synchronizing pattern followed by an identifying pattern and "blank" sections which are reserved for individual use. Each of the 160 synchronizing blocks is formed of a 2-byte synchronizing pattern followed by a 4-byte identifying pattern, followed by 60 data bytes, followed by a 4-byte inner code correcting pattern followed by another 60 bytes of data followed by yet another 4-byte inner code correcting pattern, and so on. Inner code error correction and outer code error correction are terms known to those of ordinary skill in DVTR and are described in the aforementioned text. Inner and outer code error correction are used to detect and correct digital errors, using conventional information theory, in the event of dropout or other distortion in the reproduced digital signals.
FIG. 2C provides an expanded representation of the 4-byte ID pattern included in each synchronizing block. It is appreciated that each sector, segment and field is identified by the ID pattern, and since 160 synchronizing blocks may be recorded, the number of the particular synchronizing block that is identified by this ID pattern also is provided. A similar format may be used when recording the digitized audio information and FIG. 2C illustrates that, in place of identifying the synchronizing block which contains video information, the ID pattern may identify the synchronizing block which contains audio information when that synchronizing block is recorded in audio portion 2b of a track.
Although FIG. 2A illustrates the D-1 recording format when used in the NTSC 525/60 system, this same format may be used in a 625/50 system, that is, a television system in which each frame interval contains 625 line intervals and 50 field intervals are produced each second. Most notable of the 625/50 system is the PAL system; and to record a PAL television signal in the D-1 format, twelve tracks are used to record a field interval, and each field is divided into six segments rather than the five segments of the NTSC system.
When the digital video tape shown in FIG. 2A is reproduced in a normal playback mode by the playback assembly shown in FIG. 1, the digital video signals generally are recovered accurately and a video picture of high fidelity is reproduced therefrom. However, some disadvantages arise when the apparatus of FIG. 1 is used to reproduce the digital video data during special effects modes. For example, let it be assumed that the digital video tape is played back at three times the normal playback speed. For convenience, this mode of operation is illustrated in FIG. 4 with tape 2 remaining stationary and the heads seemingly "moving" from left to right. (To conserve space in the drawings, FIG. 4 illustrates pairs of tracks rather than the individual tracks which have been described above in conjunction with FIG. 2A ) But for dynamic tracking, heads A(B) as well as heads C(D) would traverse the tape along the paths illustrated by the broken lines. However, because of dynamic tracking, each pair of heads is controlled to trace each track accurately, as represented by the solid lines.
Let it be assumed that heads A and B commence the scanning of tape 2 by scanning the tracks in which segment 0 is recorded. Thus, heads A(B) may be assumed to begin scanning at sectors 0,0(0) and 1,0(0), respectively (shown with these sector, segment and field numbers in FIG. 2A). Let it be further assumed that heads A(B) begin the scanning of segment 0 in field N (such as sectors 0 and 1 of segment 0). The operation of bimorph support 3 upon which heads A and B are mounted results in a deviation in the actual trace of these heads by an amount shown as arrow 6a. This deviation is equal to approximately 1/2 of a field interval. However, and as represented by arrow 6e, after fields N, N+1 and N+2 are scanned, the bimorph support produces a deviation in the actual trace of the heads equal to about 2 field intervals and, as scanning continues, the maximum deviation produced in the head trace is on the order of about 3 field intervals. It is appreciated that this mechanical deviation may present significant mechanical stresses to the bimorph elements and also to the drive apparatus when the video tape is played back at three times normal speed. Nevertheless, such a relatively high playback speed often is used for editing purposes, such as during video program production.
In addition, it is seen that a significant "jump" in the playback heads is provided from the end of field N to the beginning of field N+3. This jump along path 7 is needed for good picture reproduction when playing back the digital video tape at three times normal speed. However, during this track jump, information normally recorded in audio sector portions 8a and 8b are not reproduced. In the D-1 format, time codes are recorded in these portions. Hence, the significant track jump represented by path 7 results in a loss of time code data reproduction.
To overcome the loss of information that otherwise would be recovered from the audio sector portions when playing back the digital video tape at three times normal speed, it has been suggested that the rotary speed of head drum 1 be increased to 12/10 the normal rotary speed so that the playback heads scan twelve tracks in the same period that they would scan ten tracks (or one field) if operated at the lower, normal speed. FIG. 5 schematically represents the scanning of tape 2 by playback heads A, B, C and D rotated at 12/10 times normal speed. For example, if the normal rotary speed of the heads is on the order of about 5,000 rpm, this proposal contemplates an increase in the head speed of 20% for a 525/60 system and an increase in head speed on the order of about 16% for a 625/50 system (in the 625/50 system fourteen tracks rather than twelve tracks would be scanned). However, this increase in the rotary speed of the heads results in a durability problem and, moreover, since the head speed is to be increased only during special effects modes, and particularly during fast forward (or scan) modes, such selectivity increases the complexity of the controlling circuitry and software.
Recognizing the drawback associated with only two pairs of playback heads, such as illustrated in FIG. 1, the assignee of the present invention has proposed an improvement wherein the number of playback heads which are used to recover digital video data from the magnetic tape is increased, thereby overcoming the aforementioned disadvantages. A schematic representation of this embodiment is illustrated in FIG. 6 wherein the same recording heads R(A), R(B), R(C) and R(D) as were used in the FIG. 1 example are used herein; but the number of playback heads mounted on each bimorph element 3, 4 is doubled. Whereas the FIG. 1 example uses two sets of playback heads with each set consisting of two heads, the FIG. 6 example uses two sets of playback heads with each set consisting of four heads. Furthermore, two heads from one set are mounted on the same bimorph element with two heads from the other. As illustrated, playback heads A.sub.1, B.sub.1, C.sub.2 and D.sub.2 are mounted on bimorph element 3; and playback heads B.sub.2, A.sub.2, D.sub.1 and C.sub.1 are mounted on bimorph element 4. The reason for staggering these respective sets of playback heads will become apparent from the ensuing discussion.
It will be appreciated that heads A.sub.1, B.sub.1, C.sub.2 and D.sub.2 scan video tape 2 in parallel and concurrently; and the tape wrap angle about drum 1 is greater than the 180.degree. separation between the two sets of heads. Hence, heads A.sub.1, B.sub.1, C.sub.2 and D.sub.2 reproduce video data concurrently with the reproduction of video data by heads B.sub.2, A.sub.2, D.sub.1 and C.sub.1. That is, both sets of heads are in magnetic contact with tape 2 at the same time. Accordingly, in a single pass, all of the playback heads play back video data simultaneously from tape 2 because one set commences its pass as the other set nears the end of its scan.
To compare the operation of the example shown in FIG. 6 with that described above in conjunction with FIG. 1, let it be assumed that the arrangement of FIG. 6 is used to play back data when the tape is driven at three times normal speed (the same playback mode that has been discussed above in conjunction with FIG. 4). Like FIG. 4, FIG. 7 illustrates pairs of tracks rather than the ten individual tracks in a field as shown in FIG. 2A. Let it be assumed that heads A.sub.1 and B.sub.1 commence scanning segment 0 of field N (such as sectors 0 and 1 of segment 0 in field N). At the same time, heads C.sub.2 and D.sub.2 are positioned at the beginning of segment 1 of field N (such as sectors 0 and 1 of segment 1, respectively). As before, but for dynamic tracking, heads A.sub.1 -D.sub.2 would trace the path represented by the broken line in FIG. 7. However, dynamic tracking produces a deviation in the actual trace of the heads such that the heads scan the path represented by the solid line. This deviation is approximately 1/2 of a field interval, as represented by arrow 11a. As scanning continues, dynamic tracking produces an ever larger deviation in the scanning trace with the maximum deviation being on the order of about 1.5 field intervals. When compared to the maximum deviation of about three field intervals produced by the arrangement shown in FIG. 1 when playing back video data at three times normal speed, it is appreciated that the example shown in FIG. 6 produces a deviation that is approximately half that of the FIG. 1 example.
As a further improvement derived from the example of FIG. 6, since eight playback heads are used, about twelve tracks may be scanned during the time that the heads effect 2.5 revolutions at normal rotary speed. Consequently, these heads are able to scan the tracks of field N and then jump to the tracks of field N+3 without jumping or skipping the audio sector portion in the last track of field N or the first track of field N+3. That is, and as compared to the operation described above in conjunction with FIG. 4, information normally recorded in audio sector portions 8a and 8b are not skipped. Consequently, time code data which normally is recorded in these portions is reproduced by the example shown in FIG. 6 even when the video tape is played back at three times normal speed.
Other improvements attained by the example shown in FIG. 6 over the example shown in FIG. 1 will be observed by further comparisons of different operations of each example. For instance, if the example shown in FIG. 1 operates to play back video data from tape 2 at the forward, normal speed, the resultant playback operation is of the type shown in FIG. 8. As before, let it be assumed that the playback heads move from left to right. Initially, playback heads A and B of the example shown in FIG. 1 may be disposed at tracks 1 and 2 of field 0, whereafter heads C and D are disposed at tracks 3 and 4, and then heads A and B are disposed at tracks 5 and 6, and so on. During this normal, forward playback mode, field 0 is reproduced, followed by field 1, then field 2, and so on, as schematically represented in FIG. 9. Thus, during a normal, forward playback mode, the reproduced field increases monotonically. This, of course, is highly desirable and advantageous and simplifies the processing of the reproduced digital video data.
However, if the example shown in FIG. 1 is used to reproduce video data from tape 2 in the reverse mode, that is, at -1.0 times normal speed, the operation may be shown as schematically in FIG. 10. Here, it is assumed that heads A and B scan tracks 17 and 18 in field 2, then heads C and D scan tracks 15 and 16, then heads A and B scan tracks 13 and 14, and so on. For proper tracking, the bimorph elements are controlled to effect dynamic tracking of heads A(B) and C(D). In the absence of dynamic tracking, these heads would scan the traces represented by the broken lines in FIG. 10. However, because of dynamic tracking, the heads effectively scan the respective tracks as represented by the solid lines. It is recognized that this dynamic tracking produces deviations in the head traces as shown by arrows 6a and 6b. But, when heads C(D), for example, scan tracks 15, 16, it is seen that field 1 is played back during the first portion of this scan and then field 2 is played back. Hence, during a reverse playback operation, the reproduced fields appear as shown in FIG. 11. That is, data from field 2 is played back when heads A, B scan tracks 17, 18, then data from field 1 is played back when heads C, D begin the scanning of tracks 15, 16, but data from field 2 is played back once again when heads C, D complete the scanning of tracks 15, 16. Thereafter, data from field 1 is played back.
This non-monotonic change in the field (e. g. data from field 2, then field 1, then field 2, then field 1 is played back) as the boundary from one field to another is crossed introduces complexity in the processing of the reproduced video data. That is, there is a risk of interpreting video data during the brief reproduction of field 1 (i. e. when heads C, D are positioned at the beginning of tracks 15, 16) as being reproduced from field 2. Moreover, if tape 2 is being transported in a so-called shuttle mode, as by advancing and reversing the tape when searching for a desired location, the resultant unpredictable pattern in which the fields are reproduced adds further complications to the video data processing operation.
The difficulties and complexities associated with playing back digital video data from the video tape at -1.0 times normal speed is exacerbated when the tape is driven at -0.75 times normal speed. At this speed, the fields are not reproduced with uniform time durations. Rather, the pattern in which the fields are played back, and particularly the time duration of each field played back at -0.75 times normal speed may be unpredictable and may appear as shown in any one of FIGS. 12A-12E. Such unpredictability adds still further complications to the processing of the reproduced video data.
Whereas FIG. 10 illustrates the use of the example shown in FIG. 1 to play back digital video data from tape 2 at -1.0 speed, reference is made to FIG. 13 to explain how the example shown in FIG. 6 is used to play back video data at -1.0 times normal speed. In FIG. 13, the respective fields are identified by binary notation and the heads scan field 10, then field 01, then field 00, and so on. For convenience, FIG. 13 identifies each segment 0, 1, 2, 3, 4 and each field polarity 0, 1 (that is, whether the field is odd or even) in each video portion on the tape. For simplification, sector identification is not separately provided. Continuing with the convention adopted above, it is assumed that the heads are moved from right to left to effect a playback operation at -1 times normal speed. Let it be assumed that heads A.sub.1, B.sub.1, C.sub.2 and D.sub.2 begin scanning the video tape from sectors 4(1), 4(1), 0(0) and 0(0) at the boundary between field 10 and field 01. Assuming that heads A.sub.1, B.sub.1, C.sub.2 and D.sub.2 lead heads C.sub.1, D.sub.1, A.sub.2 and B.sub.2, then, as the first set of heads nears the end of its pass across the tape, heads C.sub.1, D.sub.1, A.sub.2 and B.sub.2 begin scanning the sectors 3(1), 3(1), 4(1) and 4(1), respectively. Thereafter, as these heads C.sub.1 -B.sub.2 near the end of their pass across the tape, that is, as these heads approach the end of the tracks being scanned thereby, heads A.sub.1 -D.sub.2 begin the scanning of sectors 2(1), 2(1), 3(1) and 3(1), respectively. Thus, the heads advance in the reverse direction with each pass across the tape.
FIG. 14 illustrates the sectors which are reproduced by the respective heads in the approximate time relationship at which those sectors are played back. Desirably, the digital video data played back from field to field is stored in a respective field memory for further processing. As will be described below, it often is difficult to determine when all of the segments of a given field have been reproduced and stored in the proper field memory. It also is difficult to distinguish between a segment reproduced from, for example, one even (or odd) field and a segment reproduced from the next even (or odd) field.
Whereas FIG. 14 illustrates the segments which are played back from the tape shown in FIG. 13 at -1.0 times normal speed, FIG. 15 schematically illustrates the segments which are played back at -0.75 times normal speed. Like FIG. 14, FIG. 15 represents the approximate timing relationship between sectors reproduced by heads A.sub.1 -D.sub.2 and sectors reproduced by heads C.sub.1 -B.sub.2. A comparison of FIGS. 14 and 15 indicates that the pattern of sector reproduction at -0.75 times normal speed is far more complicated than the pattern of sector reproduction at -1.0 times normal speed. Even though sector, segment and field identifying data are played back, it still is difficult to determine when a complete field has been recovered from the video tape. Furthermore, by reason of the D-1 format, sectors which are reproduced when the playback heads cross a field boundary must be rearranged to insure the integrity of a complete field. That is, a sector reproduced from, for example, field 10 should not be included with sectors reproduced from field 01. Assuming that the odd/even field identification bit is recovered from each played back sector, it also is important to make certain that a sector with field polarity ID bit 0 (for example) from field -0 is not included with the sectors reproduced from field 00.
The possibility of misinterpreting a sector last reproduced from, for example, an even field as being included in the next even field to be played back now will be explained in conjunction with FIGS. 16-18. In this discussion, it is assumed that digital video data is played back from the video tape at -1.0 times normal speed. As was discussed above in conjunction with FIG. 13, let it be assumed that heads A.sub.1, B.sub.1, C.sub.2 and D.sub.2 are positioned to begin the scanning of sectors 4(1), 4(1) of field and sectors 0(0) and 0(0) of field 10, respectively. Of course, when these heads near the end of their pass heads C.sub.1 -B.sub.2 are positioned to begin the scanning of sectors 3(1), 3(-), 4(1) and 4(1) of field 01. As the two sets of heads make successive passes across the video tape, the resultant sectors played back therefrom are as shown in FIG. 17. As before, this drawing figure represents the approximate timing relationship between the video data played back by the set of heads A.sub.1 -D.sub.2 and the video data played back by the set of heads C.sub.1 -B.sub.2.
Desirably, all of the sectors included in field 10 should be written into and read from a single field memory associated with this field and, similarly, all of the sectors included in field 01 should be written into and read from another field memory. Likewise, yet a third field memory should be provided for field 00. FIG. 18 illustrates field memories 16, 17 and 18 adapted to store fields 10, 01 and 00, respectively. It is important that a sector from one field not be misinterpreted as belonging to another field and thereby read into the improper field memory. Nevertheless, it is difficult, particularly during a reverse playback mode, to determine when a complete field has been fully reproduced from the video tape and stored in a field memory. For example, even when all of heads C.sub.1 -B.sub.2 play back sectors from field 01, sectors from preceding field 10 may be reproduced thereby. This is seen when heads A.sub.2 and B.sub.2 complete the scanning of sectors 4(1), 4(1) in the last two tracks, respectively, in field 01, whereafter these same heads scan sectors 0(0) and 0(0) which are in the same tracks but disposed in preceding field 10.
In an effort to detect accurately when all of the sectors of a preceding field have been fully reproduced, it has been observed heretofore that when a particular head, such as head A.sub.1, scans segment 2 or 3 in one field, all of the segments of a preceding field will have been played back from the video tape. This is based upon the observation that segments 2 and 3 define areas which are substantially in the central portion of a video picture, as seen in FIG. 18. Hence, by the time the central portion of the video picture is reached, it is assumed that all of the video data of the preceding field has been fully played back. Unfortunately, this is not always the case, as will now be described.
Let it be assumed that, in a reverse playback mode, a determination is made that a preceding field 10 has been fully and completely played back from the video tape by the time t.sub.1 that head A.sub.1 scans sector 2(1) of field 01, such as sector 12 shown in FIG. 16. Likewise, let it be determined that all of the sectors included in field 01 have been fully and completely recovered from the video tape by the time t.sub.5 that head A.sub.1 reaches sector 13, that is, sector 3(0), in field 00. By concluding that field 10 has been fully and completely played back at timing point t.sub.1, the writing in of data into field memory 16, the field memory assigned to field 10, thus is stopped at point t.sub.1. Thereafter, it simply is assumed that any subsequently reproduced sector having a field polarity identification bit "0" must be from field 00 because all of the sectors included in field 10 have been assumed to be written into field memory 16. Consequently, subsequently reproduced sectors having a "0" field polarity ID bit will be written into field memory 18. However, this means that sectors 0(0) and 0(0) which then are played back by heads A.sub.2 and B.sub.2, that is, sectors 14 shown in FIG. 16, are assumed improperly to be sectors from field 00. From FIG. 17, it is seen that from timing point t.sub.1 to timing point t.sub.2, sectors 14 from field 10 are played back by heads A.sub.2 and B.sub.2 and should not be stored in field memory 18, the field memory assigned to field 00. But, by reason of the technique wherein it is assumed that, at timing point t.sub.1, all of the sectors included in field 10 had been fully and completely reproduced, sectors 14 are erroneously written into field memory 18 because these sectors are erroneously interpreted as being included in field 00. Consequently, a portion of the video image of the preceding field (field 10) is superimposed upon the video image of the next field (field 00) which reduces the quality of the video image that ultimately is reproduced.
Likewise, if it is assumed that at timing point t.sub.5 all of the sectors included in field 01 have been fully and completely written into field memory 17, then any sector reproduced after that point with a "1" field polarity ID bit is assumed to be included in the next-following odd field (shown as field 11). However, it is seen from FIG. 17 that at timing point t.sub.5, heads C.sub.1 -B.sub.2 play back sectors 0(1), 0(1), 1(1) and 1(1) from field 01 and, subsequently, heads C.sub.2 and D.sub.2 play back sectors 0(1) and 0(1), respectively, also from field 01. Thus, the technique of assuming that all of the sectors included in a previous field have been fully played back when head A.sub.1 plays back segment 2 or segment 3 is susceptible to error. This is because the use of two sets of four playback heads each spans a significant number of tracks in which data from different fields may be recorded. It is, of course, recognized that the difficulty in detecting when a field of digital video data has been fully and completely played back, particularly during a reverse playback mode, is due to the shuffling of sectors recorded on the video tape. Although such shuffling has been designed to minimize errors due to dropout, the reversal in fields which are played back during a reverse playback mode often results in an erroneous conclusion that a particular sector is included in the wrong field. It is recognized that relatively complicated data processing, particularly the data processing software, may be needed to minimize such errors particularly since the field identifications played back during reverse playback modes do not change monotonically.
During reverse slow motion playback modes plural field memories are used because fields of video data are written into the memories at a rate slower than the rate at which the data is read. Hence, to prevent gaps, jumps and distortion in the video picture ultimately reproduced, a field should be stored so that it may be read out more than once, if necessary. Typically, three field memories are used to provide sufficient storage capacity for reverse slow motion playback modes. However, the read/write control over these field memories is made difficult when the so-called "reverse phenomenon" is present as these fields are played back from the video tape. Such reverse phenomenon is illustrated by the playing back of one or more sectors from field 10, then one or more sectors from field 01 and then one or more sectors from field 10 once again, as shown in FIG. 17.
When digital video data is played back from a video tape by the example shown in FIG. 6 operating at -0.75 times normal speed, fields 2, 1 and 0 are recovered in the order and with the durations illustrated in FIG. 19A. It is appreciated that the reverse phenomenon occurs because multiple heads are used to play back the sectors in reverse order (that is, in an order reversed from that in which the sectors and fields were recorded). FIG. 19B shows that field 2 is stored in field memory A, field 1 is stored in field memory B and field 0 is stored in field memory C. The arrows extending between FIGS. 19A and 19B represent the writing of sectors from each played back field into a field memory assigned to that field. Once stored in a field memory, digital video data subsequently is read therefrom; and it is, of course, appreciated that video data is read from a field memory which is not then in the process of having data written thereinto.
As shown in FIG. 19C, the field memories A, B and C are read in sequence at the read-out cycle of one field period V. The read out field period V is greater than the period V.sub.1 which represents the interval during which video data is written stably into a field memory. That is, period V.sub.1 depicts the duration that video data is played back from the video tape and written into a field memory after the occurrence of reverse phenomenon. Once digital video data is read from a field memory, as depicted in FIG. 19C, it is converted to analog form by digital-to-analog conversion and then supplied to a suitable monitor, such as a television receiver.
FIG. 19C represents the reading out of the field memories at the field period V, corresponding to normal playback speed. However, if the playback speed is changed to -0.75 times normal speed, the change in tape speed results in playing back video data with phases that are not constant. Consequently, the phases at which the field memories are read likewise are not constant and, moreover, differ from the constant read-out phases depicted in FIG. 19C. If the write-in operation of FIG. 19B is taken as a reference, the phases at which the field memories are read when the playback speed is changed to -0.75 times normal speed are shown in FIGS. 19D and 19E. These figures indicate that the use of only three field memories to accommodate normal and special playback modes may not be sufficient. This is particularly true when address outstripping occurs, that is, when a particular field memory has data read therefrom simultaneously with the writing in of data. Such address outstripping is depicted at areas 9 and 10 of FIGS. 19D and 19E. Comparing FIGS. 19B and 19D, it is seen that digital video data is written into field memory C at the same time that data is read therefrom. Comparing FIGS. 19B and 19E, it is seen that data is written into field memory B simultaneously with the reading of data therefrom. A typical solution to this address outstripping condition simply is to provide an additional field memory. However, the use of additional memory devices is expensive and adds to the complexity of read/write control.